Anionic Surfactant Systems Research Articles

Surface properties, aggregation behavior and wettability of the mixed system of fluorocarbon cationic surfactant and hydrocarbon anionic surfactant in dilute solutions

Traditional perfluorooctane sulfonic acid (PFOS) fluorocarbon surfactants endangered human health and environment. Therefore, an environmentally friendly quaternary ammonium cationic fluorocarbon surfactant (PFAS-IEt) was synthesized, and surface activities, aggregation behavior, adsorption properties and wettability of PFAS-IEt/sodium n-octyl sulfate (SOS) mixtures in dilute solutions were systematically investigated. The results demonstrated that PFAS-IEt possessed excellent surface activity in aqueous solution. Surprisingly, its surface tension was only 15.33 mN/m and critical micelle concentration (CMC) was 0.002 mol/L. It could be used as a potential substitute for PFOS. In addition, PFAS-IEt had the good synergistic effect with SOS. The PTFE plate with low surface energy could be completely wetted by the binary compound solution at low concentration (0.04 mol/L) (contact angle was 0°), which was superior to the reported long chain quaternary ammonium non-ionic fluorocarbon surfactants (MN-2C9F19 and EN-2C9F19).

Synergistic Effect of Betaines and Dialkyl Chain Anionic Surfactants on Interfacial Arrangement: A Molecular Dynamics Simulation Study.

Mixed systems of betaines and anionic surfactants can have a significant synergistic effect and greatly reduce the interfacial tension (IFT), which has attracted an extensive amount of attention. However, this synergistic effect requires an anionic surfactant and betaine molecular size matching, which limits the scope of its application. In this work, we studied three mixed systems of sodium dialkyl sulfosuccinate (AOT) and betaines with different sizes by molecular dynamics simulation and an IFT experiment and explored the interfacial behavior and synergistic mechanism of AOT in single and mixed systems. The hydrophobic tail chain center angle, average rising height of carbon atoms, stretch degree and distance between the terminal carbon atoms of AOT, and tilt angles of betaine were calculated and analyzed in detail. Simulation results showed that the hydrophobic tail chain center angle of AOT in the single system was smaller, and it tended to extend into the oil phase. After being mixed with different betaines, AOT can adjust its size according to the interfacial vacancies of different betaine systems by changing the alkyl chain orientation and forming tighter interfacial films. The IFT experiment showed that betaine/AOT mixed systems achieved a lower IFT value compared with that of the single system, indicating that AOT showed a synergistic effect with betaines with different structures. This study will be importantly instructively significant for the design and research of betaine mixed systems in crude oil exploitation.

Quick and Green Procedure for the Quantification of Chromium in an Anionic Surfactant System

This study aims to establish a facile, user-friendly, accurate, and repeatable spectrophotometric detection method for Cr(III) ions in trace quantities. The method utilises reagent 1-nitroso-2-naphthol (NNPh) in a sodium dodecyl sulphate (SDS) 1.0 % micellar solution, replacing the old, time-consuming, and expensive solvent extraction method that employs hazardous solvents such as carbon tetrachloride and chloroform. SDS surfactant is used to solubilise indissoluble chelates, making the proposed technique easy, rapid, and environmentally friendly. The established method exhibits an improved linear calibration range, sensitivity, limit of detection, and selectivity compared to earlier spectrophotometric methods employing different complexing agents. The stoichiometric ratio between Cr and NNP was observed to be 1 : 3. The molar absorption coefficient (<i>ε</i>) was determined to be 2.05 ∙ 104 l mol<sup>–1</sup> cm<sup>–1</sup> at <i>λ</i><sub>max</sub> 579 nm. The limits of detection for Cr(III) was measured as 3.49 ng l<sup>–1</sup>. A calibration linear curve was achieved in the concentration range of 0.5–5.0 μg ml<sup>–1</sup>. Results obtained showed good comparison with the results obtained by atomic absorption spectrometry. The developed method has been adapted for the analysis of Cr(III) in various alloys, pharmaceutical, environmental, and biological samples.

Exploring Surfactant Electrolyte-Porous Electrode Interactions

Microemulsions are an alternative to aqueous organic electrolytes for redox flow batteries (RFB) that can be modified without synthesis. Water, oil, and emulsifiers, when combined using optimized chemistries and compositions, produce stable mixtures with aggregate structures (microemulsions). Design limitations imposed by the interdependency of redox solubility and electrolyte conductivity are circumvented by solubilizing redox active molecules in the oil phase and supporting ions in the aqueous phase. While macroscopically homogeneous, microemulsions are biphasic at the nanometer scale, effectively decoupling solubility from conductivity. Recently, the first microemulsion RFB was demonstrated using ferrocene solubilized in an anionic surfactant system. Polarization curve analysis revealed performance limitations seemingly arising from electrolyte interactions with the cell components including porous electrodes. Further optimization requires a deeper understanding of microemulsion-porous electrode interactions.Due to the complexity of microemulsion electrochemistry, a first step towards understanding these interactions is made by investigating the relationship between surfactants and porous electrodes. Vanadium redox flow battery (VRFB) electrolytes with surfactant additives are used as a model system to explore fundamental interactions, in situ, between a surfactant-containing electrolyte and a porous electrode. Transport and kinetics are evaluated as a function of electrode, electrolyte, and surfactant properties. Redox species transport is quantified through mass transfer coefficients, using a generalization of an existing porous electrode model fit to experimental polarization curve data. Electrochemical impedance spectroscopy, interpreted using existing kinetic models, is also employed to gain insight into the effects of surfactant on electron transfer rates.

Complexation between poly (styrene-co-methacrylic acid) and polyquaternium for use in shampoo formulations

The complexation between poly (styrene-co-methacrylic acid) (P(St-co-MAA)) and a series of cationically substituted quaternary ammonium derivatives of hydroxyethylcellulose, JR and LR (polyquaternium) was investigated through the viscosity change of the complexes at different mixing ratios of the two polymers. The performance of the shampoo containing the complexes was also evaluated. The results show that the viscosity of the mixed aqueous solution increased as the ratio of (P(St-co-MAA)) to JR or LR increased. The maximum viscosity was obtained when the molar ratio (r) of St in P(St-co-MAA) to cations in polyquaternium was near 1. As the mixing ratio increased further, syneresis occurred when the charge stoichiometry between MAA and the quaternary ammonium cation was 1:1. This result reveals that cation-π electron interactions rather than electrostatic attraction play a key role in forming complexes between polyquaternium and P(St-co-MAA). The higher the molecular weight of polyquaternium is, the higher the viscosity of the complex solution. The complexes maintained a large apparent viscosity at a temperature of 70 °C, a shear rate of 1 s−1 and a NaCl concentration of 1.2 mol/L. The complexes had good tolerance to temperature, salt and shearing. The addition of the complexes in shampoo was beneficial to thickening, which is difficult in amino acid anionic surfactant systems. Compared with the shampoo containing the polyquaternium normally, the addition of P(St-co-MAA) helped to generate more flocculation earlier during washing of the hair. Unexpectedly, the wet carding performance of the shampoo was significantly better than that of commercial silicon-containing shampoo (P < 0.01). The complex has potential as a thickener and a conditioner in shampoo formulations.

Study on the bactericidal activity of dodecyl dipropylene triamine and anionic mixed surfactant systems

In this study, we anticipated that cocamidopropyl betaine (CAPB) and isomeric alcohol ethoxylates (TO) could protect the antibacterial properties of dodecyl dipropylene triamine (BDA) under the influence of anionic surfactants. Different concentrations of anionic surfactants such as sodium lauryl polyoxyethylene ether sulfate (AES) and sodium dodecyl sulfate (SDS) were added to a mixture of the cationic surfactants BDA and benzalkonium chloride (BZK) with two protective agents: a branched nonionic surfactant TO and a zwitterionic surfactant CAPB. Opticaldensity (OD) and Cell Counting Kit-8 (CCK-8) methods were used to evaluate the antibacterial activity of these systems against sulfate-reducing bacteria (SRB) such as Escherichia coli, Staphylococcus aureus, and Desulfovibrio sp. The protective mechanisms of the different protective agents were studied by measuring their respective viscosities. The protective agents (TO and CAPB) had a negative effect on the biocidal performance of the BZK (C12-C14) but had an excellent protective effect in the presence of anionic surfactants. In the BDA systems (AES and SDS concentrations of 100 mM), the BDA-CAPB-AES, BDA-CAPB-SDS, BDA-TO-AES, and BDA-TO-SDS systemseliminated almost all E.coli and S.aureus bacteria within 2 min of exposure, among which the BDA-CAPB-AES and BDA-TO-AES systems showed the strongest bactericidal activity against sulfate reducingbacteria (SRB). Therefore, these multicomponent surfactant antibacterial systems can potentially be used to replace traditional cationic surfactant systems in the chemical industry, thereby reducing the microbial corrosion caused by SRB.

Comparative study of the imbibition patterns of two types of surfactants and their residual oil morphology in low-permeability reservoirs

Forward imbibition and reverse imbibition are common imbibition modes in reservoir development. In order to understand the characteristics of recovery efficiency, sweep mode and residual oil morphology of Surfactin (reverse imbibition) and an anionic gemini surfactant (AG, forward imbibition) systems, multiscale imbibition experiments were carried out. The core and microtube experiments showed that the capillary force was dominant in the Surfactin solution and the Marangoni force was dominant in the AG solution, when the model was horizontal. In the two-dimensional pore network imbibition experiment, the recovery rate of reverse imbibition was higher than that of forward imbibition, which was consistent with the core experiment. In Surfactin solution, the dominant imbibition channel formed and swept in a fan-like pattern, however, in AG solution it failed to form a dominant imbibition channel and swept in a finger-in pattern. In the initial stage, the imbibition rate in Surfactin solution was significantly higher than that of AG solution, and its sweep efficiency was 1.5 times higher than that of AG solution. In terms of the morphology of residual oil, less oil remains on the wall during reverse imbibition, and columnar and cluster residual oil was easy to form in the channel. After long-term molecular diffusion, the Marangoni effect can effectively improve mixing between the oil and water phases, thus improving the sweep efficiency. In the case of forward imbibition, there were more residual oil films on the solid wall and punctiform residual oil was easily formed in the channel. These discoveries provide a basis for the selection of imbibition fluids or the optimization of oil recovery processes in reservoir development.

Molecular dynamics simulation of self-assembly in anionic and zwitterionic surfactant systems

Molecular dynamics simulation of self-assembly in anionic and zwitterionic surfactant systems

Dissipative particle dynamics simulation and experimental analysis of the effect of anionic/cationic mixed surfactants on the stability of emulsions

Hydrogen bond shaping plays a very important role in the stability of emulsions and cannot be ignored when developing highly stable emulsions. Mixed systems of anionic and cationic surfactants can greatly reduce interfacial tension. In this study, we introduced different amounts of hydroxyl groups in Gemini surfactant spacers through dissipative particle dynamics simulations and evaluated the effect on the oil–water interface. In addition, Gemini cationic surfactants were mixed with anionic surfactants and the emulsification effect of the mixed system was evaluated, as well as the effect on the emulsion-formation and emulsion-breaking processes. We found that the anionic and cationic surfactants have a strong synergistic effect that significantly reduces interfacial tension and effectively transforms the oil–water interface into an emulsion state, but the effect of the interaction between the anion and the cation is affected by the spacer group. This work provides theoretical guidance and ideas for designing efficient emulsifiers and improving emulsion stability in the future.

Study on the construction of anionic-cationic surfactant binary mixed system and its wettability alteration performance in tight reservoirs

To solve the problem of reducing reservoir permeability in tight reservoirs due to higher capillary forces, a method is proposed to change the reservoir wettability by using surfactants and then increase oil and gas recovery rates. Mixed systems of anionic and cationic surfactants are less used in wettability alteration due to strong aggregation behavior. However, it has been shown that anionic-cationic surfactants exhibit excellent synergistic effects at appropriate concentrations and ratios, which have good theoretical value for practical applications. In this paper, firstly, we investigated the synergistic effects of mixed system DTAB/SDS and DTAB/AES in reducing the surface tensions. It was found that there was a safe zone between the precipitation boundary and the critical micelle concentration for anionic and cationic surfactant mixed systems, which indicated that the mixed systems had a better synergistic effect without aggregation at a certain concentration and ratio. The DTAB/AES mixed system could reduce the surface tension to a low value of 18.67mN/m. Further, the wettability alteration of the mixed system was studied. Results showed that a contact angle of the sandstone altered from 10.75° to 71.8° after treating in the DTAB/SDS mixed system at a low concentration, which was significantly better than a single surfactant on performance. The total concentration of the optimum mixed system was 5 × 10−5 mol/L, and the molar ratio was 0.7:0.3. The optimum mixed system had excellent wettability alteration performance, it could significantly reduce capillary forces and relieve aqueous phase trapping damage, further, the oil and gas flow channels could be restored.

Stabilization of foam using hydrophobic SiO2 nanoparticles and mixed anionic surfactant systems in the presence of oil

The use of nanoparticles has been intensively investigated to improve foam flooding in oil production. Because nanoparticles have larger adhesion energy at the air-water interface than the surfactant, they help to prevent a gas diffusion between bubbles that leads to foam film rupture. In this study, the detail investigation on the behavior of mixed-surfactant foam stabilized by silica nanoparticles (SiO2-NPs) was conducted. The binary surfactant systems of an internal olefin sulfonate (IOS) and sodium polyethylene glycol monohexadecyl ether sulfate (C32H66Na2O5S) were selected. The impact of alkane oil with varying carbon chain length (i.e. hexane, octane, dodecane, and hexadecane) was investigated. The findings from a similar system of foam in deionized water (DI) and brine solution also presented for comparison. For SiO2-NPs foam/brine system in the presence of oil, the foam stability increased to a maximum with respect to certainly added nanoparticles (10 ppm), and the additional surfactant is required to maintain the foam stability when the alkane chain length increases. The foam stability parameter indicated by spreading (S) and entering (E) coefficients shows that the SiO2-NPs foam systems with the shorter-tail surfactant (mixed C15-18 IOS) had lower the coefficient values compared to the longer-tail one (mixed C19-23 IOS), suggesting that the structure of surfactant influents on foam stability. The findings from this study bring an understanding of the SiO2-NPs foam/alkane oil behavior in the brine solution, which helps to select the appropriate system for improving foam flooding in EOR applications.

Quantitative analysis of micellar effect on the reaction rate of cationic triphenylmethine dyes with water according to Berezin’s model

Several approaches quantitatively describe the effect of surfactant micellar solution on the reaction rate. The most used among them are Piszkiewicz’s, Berezin’s, and Pseudophase Ion-Exchange (PIE) models. The last-named was developed by Bunton and Romsted. Piszkiewicz’s model is based on representations of the micellization according to the mass action law with the formation of a catalytic micelle, which consists of some surfactant molecules and a substrate. In our previously paper, this model was used to explain the kinetic micellar effect on the reaction of cationic triphenylmethine dyes with water once again showed the main disadvantages of this approach. Berezin’s model is based on another model of micelle formation viz. the pseudophase model, and the binding of reagents by micelles is considered as the distribution of a substance between two phases. In this work, we aim to consider the applicability of Berezin’s approach for the interaction of malachite green and brilliant green cations with water molecule as a nucleophile in aqueous systems of nonionic, anionic, cationic, and zwitterionic surfactants. On the whole, Berezin's model performed well when applied to the description of the micellar effect on the reaction of similar dye with the hydroxide ion. However, it was revealed that this model does not take into account the change in the local concentration of the HO– ions due to a compression of the double electric layer upon addition of reacting ions to the system, as well as the constant of association of the HO– ions with cationic head groups of surfactant. In this case, when water is used as a nucleophile, the question of the degree of nucleophile binding can be solved differently. The PIE model is also based on a pseudophase model of micellization, but a substrate binding by micelles is considered as an association in a stoichiometric ratio of 1:1, and a nucleophile concentration is expressed in a local concentration based on the neutralization degree of micelles. Given the latter, its approach cannot be applied to the kinetic micellar influence on the reaction of cationic triphenylmethine dyes with water.

Aqueous Foam Stabilized by Hydrophobic SiO2 Nanoparticles using Mixed Anionic Surfactant Systems under High‐Salinity Brine Condition

AbstractA primary concern of surfactant‐assisted foams in enhanced oil recovery (EOR) is the stability of the foams. In recent studies, foam stability has been successfully improved by the use of nanoparticles (NP). The adhesion energy of the NP is larger than the adsorbed surfactant molecules at the air–water interface, leading to a steric barrier to mitigate foam‐film ruptures and liquid‐foam coalescence. In this study, the partially hydrophobic SiO2 nanoparticles (SiO2‐NP) were introduced to anionic mixed‐surfactant systems to investigate their potential for improving the foamability and stability. An appropriate ratio of internal olefin sulfonate (C15‐18 IOS) and sodium polyethylene glycol monohexadecyl ether sulfate (C32H66Na2O5S) was selected to avoid the formation of undesirable effects such as precipitation and phase separation under high‐salt conditions. The effects of the NP‐stabilized foams were investigated through a static foam column experiment. The surface tension, zeta potential, bubble size, and bubble size distribution were observed. The stability of the static foam in a column test was evaluated by co‐injecting the NP‐surfactant mixture with air gas. The results indicate that the foam stability depends on the dispersion of NP in the bulk phase and at the water–air interface. A correlation was observed in the NP‐stabilized foam that stability increased with increasing negative zeta potential values (−54.2 mv). This result also corresponds to the smallest bubble size (214 μm in diameter) and uniform size distribution pattern. The findings from this study provide insights into the viability of creating NP‐surfactant interactions in surfactant‐stabilized foams for oil field applications.

Study of aggregation behavior between N-lauryl sarcosine sodium and Dodecyltrimethylammonium bromide in aqueous solution, using conductometric and spectrophotometric techniques

The micellization behavior of N-Lauryl sarcosine sodium (NLS, an anionic surfactant) and Dodecyltrimethylammonium bromide (DOTAB, a cationic surfactant) in aqueous solution is investigated by UV–vis spectrophotometric and conductometric methods at 298.15 K. A critical micelle concentration (CMC) were obtained for pure NLS and DOTAB as well as mixed micelles [NLS(1) + DOTAB(2)]. The CMC values of the studied system at different mole fractions (yi) are much less than that of individual surfactants (due to the strong electrostatic attraction between components after mixing). In this work, the tautomers of 1-phenyl-1, 3-butadione (or benzoylacetone) were used to obtain the mixed CMC of anionic surfactant systems by UV–Vis technique. The deviations of critical micelle concentration (CMC) from the ideal values show the interaction between NLS and DOTAB. The micellar mole fraction values (X1m and X2m) according to Rubingh approach were calculated and the results obtained reveal the same composition in the mixed micellization at most mole fraction(y1) even when the initial mixing ratio is far from equimolar. The activity coefficients of components were less than unity (f1m < < 1 and f2m < < 1) and the values of the interaction parameter (βm) are negative in all compositions, which indicate the strong synergistic interaction between NLS and DOTAB. The Gibbs free energy of micellization (ΔGmic0) and the excess free energy of micellization (GmicE) values are found to be negative in this system demonstrated the stability of the mixture. All of the evaluated results imply a strong synergism between components.

Mechanisms of anionic surfactant penetration into human skin: Investigating monomer, micelle and submicellar aggregate penetration theories.

Once penetrated into the stratum corneum, anionic surfactants bind to and denature stratum corneum proteins as well as intercalate into and extract intercellular lipids. With repeated exposures, this leads to skin dryness and irritation, compromising barrier function and skin health. The mechanisms of anionic surfactant penetration into the skin, however, are still widely debated. The objective of this study was to evaluate current theories of surfactant penetration into human skin. A test set comprising 15 anionic surfactant systems and one non-ionic surfactant, all having either dodecyl or lauryl alkyl chains, was tested for surfactant penetration into split-thickness human cadaver skin invitro using radiolabelled sodium dodecyl sulphate (14 C-SDS). Select physical properties of these formulations thought to be associated with skin penetration including critical micelle concentration, micelle diameter, filtrate concentration and zeta potential were also measured. 14 C-SDS penetration into human cadaver skin from surfactant systems invitro was found to correlate well with CMC (R2 =0.34, P<0.05), filtrate concentration (R2 =0.36, P<0.05) and zeta potential (R2 =0.76, P<0.001), but poorly with micelle diameter (R2 =0.12). Furthermore, the latter measure correlated inversely with penetration compared to what would be expected based on the micelle penetration theory. Neither monomer nor micelle penetration theories are sufficient to explain anionic surfactant penetration into human skin. Submicellar (or premicellar) aggregate penetration theory is difficult to defend at relevant surfactant concentrations. We propose a new hypothesis for this mechanism in which short-term penetration is based on monomer concentration and longer term penetration is based on surfactant-induced damage to the skin barrier.

Synergistic effect of mixed anionic and cationic surfactant systems on the interfacial tension of crude oil-water and enhanced oil recovery

Surfactant based enhanced oil recovery (EOR) is an interesting area of research for several petroleum researchers. In the present work, individual and mixed systems of anionic and cationic surfactants consisting of sodium dodecyl sulphate (SDS) and cetyltrimethylammonium bromide (CTAB) in different molar ratios were tested for their synergistic effect on the crude oil-water interfacial tension (IFT) and enhanced oil recovery performance. The combination of these two surfactant systems showed a higher surface activity as compared to individual surfactants. The effect of mixed surfactant systems on the IFT and critical micellar concentration (CMC) is strongly depends on molar ratios of the two surfactant. Much lower CMC values were observed in case of mixed surfactant systems prepared at different molar ratios as compared to individual surfactant systems. The lowest CMC value was found when the molar concentration of SDS was higher than the CTAB. When the individual and mixed surfacant systems were tested for EOR performance through flooding experiments, higher ultimate oil recovery was obtained from mixed surfactant flooding compared to individual surfactants. Combination of SDS and CTAB or probably other anionic-cationic surfactants show synergism with substantial ability to reduce crude oil water IFT and can be a promising EOR method.

Properties of quaternary ammonium surfactant with hydroxyethyl group and anionic surfactant mixed systems

The surface activities and application properties for the mixtures of cationic surfactant didecylmethylhydroxyethylammonium acetate (DEAQ) and anionic surfactant sodium fatty alcohol ether carboxylate (AEC9-Na) were studied systematically. The interaction parameters and thermodynamic micellization parameters of DEAQ/AEC9-Na mixtures were evaluated. Results showed that the mixed systems have higher surface activity, lower contact angle and better wetting ability than either component due to the synergistic effect and there are attractive interactions between molecules for the surfactant mixtures at the mixed micelles and the air/liquid interfaces. The thermodynamic parameters indicate that the micellization of the mixtures is an entropically driven process. When the mole fraction of DEAQ (αDEAQ) is 0.608, the emulsifying capacity for liquid paraffin and soybean oil of DEAQ/AEC9-Na mixtures is the best among the mole ratios studied, and the emulsifying capacity of the mixtures is stronger than that of AEC9-Na except αDEAQ = 0.509. For the mixed systems, the foaming ability is the best when αDEAQ = 0.608, whereas the foam stability is the best when αDEAQ = 0.280. And there were greater foaming abilities and foam stabilities for mixed systems than those of DEAQ.

Mixed surfactant enabled EVA emulsion for PPD applications

PPD emulsion product is advantageous for use in sub-ambient temperature as it improve the physical handling characteristic compared to traditional product. However, existing PPD emulsion system is solely implementing a single anionic surfactant system which leads instability towards a series of temperature changes. Therefore, mixed surfactant (anionic and non-ionic) surfactant was introduced to overcome this problem. Through this research, sodium dodecyl sulfate (SDS) act as anionic surfactant and Tween 80 used as non-ionic surfactant in this study. The stability of an emulsion was identified through particle size and zeta potential. For the mixed surfactant composition, the result show the presence of SDS content contributes in reduction on particle size. Furthermore, the study on effectiveness of surfactant on EVA emulsion was evaluated. The result indicates with present of surfactant, interfacial surface tension was reduced. The freezing point and pour point depressant test of the EVA emulsion was evaluated. Present of non-ionic surfactant help in stability and flow-respond at low temperature. Mixed surfactant system provides sufficient protection from droplet size growth caused by the temperature changes which eventually leads to instability.

A responsive anionic wormlike micelle using pH-directed release of stored sodium based on polybasic acids.

Responsive wormlike micelles are very useful in a number of applications, whereas it is still challenging to create dramatic viscosity changes in anionic surfactant systems. Here a differential pH-responsive wormlike micelle based on sulfonic surfactants was developed, which is formed by mixing sodium dodecyl trioxyethylene sulphate (SDES) and ethylenediaminetetraacetic acid tetrasodium (EDTA4-·4Na+) at the molar ratio of 1 : 1. The phase behavior, aggregate microstructure and viscoelasticity of the SDES/EDTA4-·4Na+ solution were investigated via macroscopic observation, cryo-TEM and rheological measurements. It was found that the phase behavior of the SDES/EDTA4-·4Na+ solution undergoes transitions from a water-like fluid to viscoelastic upon decreasing the pH. On decreasing the pH from 12.01 to 3.27 by adding HCl, the viscosity of the transparent solutions with wormlike micelles was increased rapidly and reached ∼3100 mPa s. Furthermore, on increasing the pH by adding NaOH, the viscosity was slightly increased due to the addition of Na+. However, the increase in the concentration of Na+ is much smaller than the theoretical addition. The same phenomenon was noted in the sodium citrate solution, but does not exist in the sodium formate system. The viscosity of the micellar solution has a sensitive response to inorganic acids and tolerance to inorganic bases due to the characteristics of polybasic acids.

Phase Diagram of Monohexadecyl Phosphate Neutralized by L-Arginine: α-Gel Formation Ability.

The aqueous phase diagram of monohexadecyl phosphate neutralized by L-arginine (C16MP-Arg) was studied using analytical techniques including differential scanning calorimetry (DSC) and small/wide-angle X-ray scattering (SWAXS) measurements. We found that the mixture could form an α-gel over wide ranges of concentrations and temperatures. The α-gel was thermodynamically stable at 25°C (below the phase transition temperature for hexagonal or lamellar liquid crystals), and did not transform to coagel for at least one year. This behavior is unique among anionic surfactant systems. Thus, the balance between hydrophilic and hydrophobic volumes is a key factor for determining whether a stable α-gel can be obtained. In this system, the relatively large volume of Arg relaxed the alkyl chains of C16MP and prevented their crystallization.